The invention relates to systems and methods for communications of digit ally modulated signals, and more particularly to systems and methods utilizing an amplitude-modulation (AM) frequency band to communicate digitally modulated signals.
The explosive growth of digital communications technology has resulted in an ever-increasing demand for bandwidth for communicating digital information. Because of the scarcity of available bandwidth for accommodating additional digital communications, the industry recently turned its focus to the idea of utilizing the preexisting analog AM band more efficiently to help make such an accommodation. However, it is required that any adjustment to the AM band to provide the additional capacity for digital communications does not significantly affect the analog AM signals currently generated by radio stations on the same band for AM radio broadcast. In the United States, adjacent geographic areas covered by AM radio broadcast are assigned different AM carrier frequencies, which are at least 20 kHz apart. Specifically, when they are exactly 20 kHz apart, the AM carrier assigned to the adjacent area is referred to as a xe2x80x9csecond adjacent carrier.xe2x80x9d
An in-band on channel AM (IBOC-AM) (also known as xe2x80x9chybrid IBOC-AMxe2x80x9d) scheme utilizing bandwidth of the AM band to communicate digital audio information has been proposed. In accordance with the proposed scheme, digitally modulated signals representing the audio information populate, e.g., a 30 kHz digital band centered at an analog host AM carrier. The power levels of the spectrums of the digitally modulated signals are allowed to e equally high across a 10 kHz subband in the digital and on each end thereof.
However, in implementation, it is likely that two such IBOC-AM schemes would be respectively employed in two adjacent areas, to which the host AM carriers assigned are 20 kHz apart. In that case, the 30 kHz digital bands for digital communications entered at the respective host AM carriers overlap each other by 10 kHz, thereby causing undesirable xe2x80x9cadjacent channel interferencexe2x80x9d to each area. In particular, such interference is referred to as xe2x80x9csecond adjacent channel interference,xe2x80x9d as the dominant interfering carrier in this instance consists of a second adjacent carrier. The second adjacent channel interference degrades the digital communications in each of the adjacent areas, especially in the parts of the areas which are close to their common border.
Accordingly, there exists a need for a technique for effectively reducing adjacent channel interference in adjacent areas where IBOC-AM schemes are employed.
The invention overcomes the prior art limitations by using improved power profiles for transmission of digitally modulated signals to reduce the adjacent channel interference described above. In accordance with the improved power profiles, the power levels of a subset of the digitally modulated signals populating a subband (e.g., the overlapping 10 kHz subban described above) in the aforementioned digital band are made relatively low, with respect to the power levels of a second subset of the digitally modulated signals populating a second subband (e.g., the entire digital band excluding the overlapping band) in the digital band.
In accordance with an aspect of the invention, at least one of the digitally modulated signals represents control information indicative of a selected one of the improved power profiles which is used in the transmission. This stems from the fact that such control information needs to be transmitted to a receiver for proper recovery of the digital information represented by the remaining digitally modulated signals.